1. Field of the Invention
The present invention relates to a lens optical system, and more particularly to a lens optical system that employs a cemented lens element having a diffraction grating.
2. Description of the Prior Art
A lens element that has a light-condensing ability offered by a diffraction grating formed thereon (hereafter such a lens element will be referred to as a diffractive lens element) has useful optical properties that are not found in a well-known refractive lens element. For example, a diffractive lens element offers the following advantages. First, by providing a diffractive lens element on a lens surface of an ordinary refractive lens element, it is possible to give a single lens element both light-diffracting and light-refracting abilities. Second, a diffractive lens element makes effective correction of chromatic aberration possible because, in it, the quantity that corresponds to the dispersive power of a refractive lens element has the opposite sign.
Accordingly, by providing a diffractive lens element on a lens surface of a refractive lens element, it is possible to correct chromatic aberration, which is conventionally corrected by the use of a combination of two, a positive and a negative, refractive lens elements, by the use of a single lens element. Although a diffractive lens element has useful properties as described above, it also suffers from problems resulting from the fact that the diffraction efficiency of the diffraction grating is wavelength-dependent. For example, except at the design wavelength, the diffracted light of orders other than the intended order is too intense. This causes ghosts, and thereby degrades imaging performance. In particular, an optical system designed for white light, i.e. one that needs to cope with a wide range of wavelengths, suffers greatly from this problem.
To solve this problem, U.S. Pat. No. 5,847,877 and a report written by Steven M. Ebstein (the Sep. 15, 1996 issue of Optical Society of America) each propose a diffractive optical element of the type that has a relief pattern constituting a diffraction grating formed at the cementing interface between two different optical materials. Here, by exploiting the fact that the difference in refractive index between two optical materials depends on the wavelength, the wavelength-dependent variation in phase difference is successfully prevented, and thereby higher diffraction efficiency is achieved over a wide wavelength range.
However, to realize such a diffraction grating, a couple of requirements as noted below need to be satisfied.
First, to obtain sufficiently high diffraction efficiency, the diffraction grating needs to be composed of an optical material that has a relatively high refractive index and a relatively low dispersion and an optical material that has a relatively low refractive index and a relatively high dispersion.
Second, it is necessary to make the blaze vertex angle as great as possible. When the blaze shape of a diffraction grating is manufactured by molding, the larger the blaze vertex angle, which is determined in terms of the grating height and the grating pitch of the diffraction grating, the more easily the material can be filled up to the very tip of the blaze vertex. Thus, the larger the blaze vertex angle, the more accurately the blaze shape can be transferred (i.e. molded). Test manufacturing revealed that, by setting the blaze vertex angle to be around 70°, it is possible to achieve satisfactorily accurate transfer (i.e. molding).
Hereafter, the second requirement noted above will be described in more detail with reference to FIG. 11 illustrating a blaze shape. In FIG. 11, Θ represents the blaze vertex angle; h0 represents the grating height (i.e. the trough-to-ridge height) of the diffraction grating; and d represents the grating pitch. If it is assumed that the refractive index of the medium that exists on the incident side at the design wavelength λ0 is n0, and that the refractive index of the medium that exists on the exiting side at the design wavelength λ0 is n′0, then the grating height (h0) of the diffraction grating is defined by the formula: h0=λ0/(n0−n′0).
The grating pitch (d) represents the intensity of the light-diffracting action. The smaller the grating pitch (d), the stronger the light-diffracting action. That is, in a diffractive lens element, the smaller the grating pitch (d) it has, the stronger the diffractive power it exerts. Accordingly, if a diffractive lens element is used to correct the chromatic aberration caused by a lens element made of high-dispersion glass, or by a lens element having a strong optical power, the diffractive lens element needs to be given a comparatively strong diffractive optical power. Moreover, since a diffractive lens element needs to be so shaped that its light-diffracting action becomes stronger gradually from the center to the edge, the larger the effective diameter of the lens element, the smaller the grating pitch (d) needs to be at the edge of the lens element.
Furthermore, the larger the blaze vertex angle (Θ), the more easily the material can be filled up to the very tip of the blaze vertex. This makes accurate transfer (i.e. molding) of the blaze shape possible. The lower the grating height (h0) of the diffraction grating, and the greater the grating pitch (d), the greater the blaze vertex angle (Θ) can be made. However, to satisfy the first requirement noted above, the diffraction grating needs to have a grating height (h0) as high as about 6 to 17 μm. Accordingly, if chromatic aberration is corrected solely by a diffractive lens element, in order to obtain a sufficiently strong diffractive optical power as required, the diffractive lens element needs to have an unduly small grating pitch (d) and thus an unduly small blaze vertex angle (Θ). This degrades the transfer accuracy of the blaze shape.
Next, how chromatic aberration is corrected by means of a diffractive lens element will be described. In general, chromatic aberration is corrected in such a way that the imaging position of light having the wavelength of the F line coincides with the imaging position of light having the wavelength of the C line. In this case, however, the imaging position of light having the wavelength of the F line deviates from the imaging position of light having the wavelength of the d line, and simultaneously the imaging position of light having the wavelength of the d line deviates from the imaging position of light having the wavelength of the C line. This is called the secondary spectrum. The secondary spectrum tends to be larger where chromatic aberration is corrected by the use of a diffractive lens element than where chromatic aberration is corrected by the use of a combination of two, a positive and a negative, refractive lens elements. To minimize also the secondary spectrum, it is preferable that chromatic aberration be corrected by the use of both a diffractive lens element and a combination of two, a positive and a negative, refractive lens elements.